Rough grinding refers to a method of roughly processing metal under severe cutting conditions. To minimize forces applied during rough grinding, the cutting edges of a cutting insert are provided at a high position and the grooves on the inside are typically formed to slope down towards a center of the cutting insert.
In a single-sided cutting insert, the supporting surface at the bottom portion is ground flat so that it can be securely mounted on a tool holder or a reinforcing part (i.e., a ‘shim’). However, in a double-sided cutting insert wherein its top and bottom are symmetrical to each other, the supporting surfaces of such cutting insert are located at a lower position than the cutting edges. Consequently, the supporting surfaces cannot be ground and the inevitable deformations occurring during a manufacturing process tend to cause deviations in the level of the supporting surfaces. In such a case, the cutting insert cannot be securely mounted on the reinforcing part or the tool holder.
Korean Patent Application No. 2008-7026943 is directed to mounting a double-sided indexable cutting insert (1) on a supporting plate (2). FIG. 1 illustrates a tool holder and a supporting plate in accordance with the prior art. The upper portion of the supporting plate (2) is provided with a plurality of planar zones (3a, 3b, 3c), while the upper surface of the cutting insert (1) is provided with contact surfaces (4a, 4b, 4c) that contact the planar zones. The contact surfaces of the cutting insert are formed by machine pressing. However, due to the inevitable deformations of the cutting insert during a pressing process and the resulting deviations in the height of the supporting surfaces, only two of the contact surfaces might contact the planar zones and a gap may exist between the remaining contact surface and the planar zone. This causes the cutting insert to vibrate during cutting processes, which in turn significantly compromises accuracy, and substantially reduces the cutting tool's lifespan.
Additionally, a through-hole is formed at the center of the above cutting insert for mounting the cutting insert on the reinforcing part or the tool holder. When the cutting insert is mounted by means of a screw, the screw is inserted into the tapered through-hole of the cutting insert. In such a case, since the screw engages the internal thread of a screw that is used to mount the supporting plate, the clamping force needed to secure the cutting insert is limited. Further, replacing the cutting insert becomes cumbersome since the screw must be fully disengaged each time the cutting insert is replaced. Moreover, the screw easily wears out, thus reducing the clamping force that pushes the cutting insert against the wall of the tool holder, which in turn often causes damages to the cutting insert. When the cutting insert is mounted by means of a lever, the through-hole of the cutting insert is prismatic, i.e., the diameter of the through-hole is constant. While this provides a good clamping force against the wall of the tool holder, the clamping force against the bottom of the tool holder's pocket tends to be poor, thereby causing the cutting insert to lift during cutting processes and as a result becoming easily damaged. Accordingly, mounting the cutting insert by means of a screw or a lever causes the problem of the cutting insert becoming dislodged from the tool holder's pocket under severe cutting conditions.